LETTERS TO SANTA (48A. Certain holiday mail … or what 20-, 25- and 43-Across have in common) along with SANTA in circled letters found in SECRET AGENT MAN (20A. 1966 Johnny Rivers hit),SLAM ON THE BRAKES (25A. Create skid marks, perhaps) and SEASONED VETERAN (43A. One with lots of experience) constitute the interrelated group of this Tuesday crossword.